VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. SOS 



the lights perfectly soft and gentle, that motion produces little or no effect on 

 the lights themselves, either to increase or diminish their brilliancy. 



These tables, which are 10 inches wide and 35 inches high, and the one of 

 them 12 feet, and the other 10 feet long, are placed at an angle of 6o° from 

 each other, and in such a situation with respect to the photometer, that lines 

 drawn through their middles in the direction of their lengths, meet in a point 

 exactly under the middle of the vertical plane or field of the photometer, and 

 from that point the distances of the lights are measured ; the sides of the tables 

 being divided into English inches, and a vernier, showing lOlhs of inches, being 

 fixed to each of the sliding carriages on which the lights are placed. These 

 carriages are so contrived that they can be raised or lowered at pleasure, which 

 is absolutely necessary in order that the lights may be always of a proper height, 

 namely, that they may be in a horizontal line with the tops of the cylinders of 

 the photometer. In order that the 2 long and narrow tables or platforms, just 

 described, on which the lights move, may remain immoveable in their proper 

 positions, they are both firmly fixed to the stand which supports the photometer; 

 and in order that the motion of the carriages which carry the lights may be as 

 soft and gentle as possible, they are made to slide on parallel brass wires, Q inches 

 asunder, about -pV of an inch in diameter, and well polished, which are stretched 

 on the tables from one end to the ether. 



The pane of glass covered with white paper, which being fixed in a groove in 

 the back of the box, constitutes the vertical plane on which the shadows are 

 projected, is 5^ inches long, and 3^ inches wide, as has already been observed ; 

 which is much larger than the dimensions assigned above for the field ; namely, 

 1-rV inches wide, and 2 inches high. I had two objects in view in this arrange- 

 ment ; first, to render it easier to fix this plane in its proper position ; and 2dly, 

 to be able to augment occasionally the dimensions of the field, by removing en- 

 tirely the black pasteboard screen from before this plane, or making use of an- 

 other with a large aperture ; which is sometimes advantageous *. 



The first attempts in the experiments, were to determine how far it might 

 be possible to ascertain, by direct experiments, the certainty of the assumed law 

 of the diminution of the intensity of the light emitted by luminous bodies ; 

 namely, that the intensity of the light is every where as the squares of the dis- 

 tances from the luminous body inversely. These experiments appeared the 

 more necessary, as it is quite evident that this law can only hold good when the 



* Since writing the above, I have made a little alteration in the form of the box which contains 

 the photometer. The front of it, instead of being open, is now closed, and the light is admitted 

 tlirough 2 horizontal tubes, which are placed so as to form an angle of 6i/°; their axes meeting at 

 the centre of the field of the instrument. The field of tlie photometer is viewed tlirough an opening 

 made for that purpose in the middle of the front of the box, between the two tubes abovementioned. 



